Damage to electrical and electronic equipment by lightning may be caused by one or more mechanisms. A direct strike may lead to potentially explosive mechanical failure. An electromagnetic pulse resultant from lightning may induce excessive voltages and currents in un-shielded conductors, including those inside integrated circuits. Wherever electronic equipment is powered remotely, a strike may lead to a ground potential rise causing a high potential difference on equipment terminals relative to a remote reference, either some distance away horizontally along the ground or between the top and bottom of a tower.
In respect of lightning strikes on a tower, the probability of a strike increases exponentially with height of location of the tower and height of the tower, ranging from tens of strikes per year on a hilltop to one or two strikes per year at sea level. Lightning strike currents range in intensity from 1-250+kA peak amplitude.
A number of measures to provide lightning protection for electronic equipment mounted on towers are presently used. Air terminals aim both to help prevent direct equipment strikes, using a “cone of protection” or “rolling sphere” concept, and to capture main strike energy when a strike occurs. Effective grounding of a tower structure channels a main current flow to earth and minimises surge currents in cables passing up and down the tower to service equipment mounted on the tower. Electromagnetic pulse shielding of sensitive equipment is provided by overall metal enclosure or use of localised metal “cans” as Faraday cages. Surge arrestor devices aim to limit voltage rise on equipment terminals and allow a large proportion of the surge current to flow between signal and ground conductors to equalise potential.
In respect of surge arrestor devices, use of voltage-activated devices is known to trip and short circuit a lightning surge to earth, for example, devices such as gas discharge tubes, metal oxide varistors the resistance of which drops above a predetermined voltage or Zener diodes. A requirement is that with lightning strikes producing currents of 1-250+kA with rapid rise and fall times, protection devices need to react quickly and to be able to handle large currents. However, although gas discharge tubes are reasonably quick and cope with short-term high currents, they have limited lifetimes dependant on a number and size of strikes. Such lightning protective devices may be rendered ineffective by a single large strike. Unless the strikes are counted and measured to predict when the protective devices need replacing, the devices have to be replaced on a conservative fixed time schedule to maintain protection. That is, the use of surge protection devices is undesirable because their high current handling capability comes at a price of a limited life so that it is necessary to replace the surge protection devices, for example field replaceable gas discharge tubes, on a regular basis depending on local lightning strike frequency. A risk remains that a level of protection provided may not be sufficient or that multiple strikes in a short period of time could destroy expensive equipment and lead to extended service outages.
An example of tower-mounted electronics equipment is wireless communications equipment mounted on a tower, although the invention is not limited to the protection of such equipment.
A low noise amplifier is typically provided at the top of a wireless communications tower, such as a cellular tower, to boost receive signals, thereby to increase cell radius. DC power is supplied to the amplifier via an up-signal coaxial cable, with a high-frequency pass filter to pass gigahertz frequency up-signals but filter out lightning frequencies which have energy concentrated below 1 MHz, that is, the cable is filtered to pass up-signals but to filter out lightning-induced currents. However, with a same cable carrying up-signals and DC supply, a simple high-pass filter cannot be used because a DC path is also needed. Instead of a simple high-pass filter, a coax parallel plate capacitor may be used with low pF capacity to pass GHz frequency signals but with a DC tap protected by gas discharge tubes and/or semiconductor protective devices on a DC lead-off. Alternatively, in-line coax protection may be provided by power supply protectors, available from, for example, PolyPhaser Technical Department, PO Box 9000, Minden, Nev. 89423-9000, incorporating metal oxide varistors. These are hybrid devices on the DC route with a capacitor blocking device.
It would be preferable to locate further active electronics equipment at a top of a mast with a power supply line but no RF path up the tower. Active electronics equipment located at the top of tower has an advantage that there is no requirement for expensive, large-diameter, low-loss coaxial cable to provide an RF path from a base to the top of the tower. Moreover, a power amplifier does not need to allow for loss in such a cable if located at the masthead.
There remains, then, a problem of providing lightning surge protection for the power line to power masthead-mounted electronics equipment without regularly replacing surge protection devices, while providing a communications link to the masthead equipment.
In summary, remote powering of electronics equipment is problematic because lightning surges produce potential differentials between opposed ends of a power supply line. Known remote powering solutions include DC bias on coaxial cables used for powering of satellite dish low noise block and masthead low noise amplifiers; pseudo square wave AC on coaxial cables, which is common practice in North American cable TV networks for powering distribution nodes; power over Ethernet DC, in which power is transmitted on spare twisted pair conductors. However, all of these are unbalanced transmissions, referenced to a local earth and are vulnerable to ground potential rise. Power over Ethernet is not a true floating power without DC/DC conversion, but instead reliance is placed on shielding and on any induced signals being well-matched and therefore not producing a damaging potential difference at the remote equipment.